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<title>e常见约束和约束作用</title><link href='file://night/mermaid.dark.css' rel='stylesheet' type='text/css' /><link href='file://night/codeblock.dark.css' rel='stylesheet' type='text/css' /><link href='file://night/sourcemode.dark.css' rel='stylesheet' type='text/css' /><style type='text/css'>html {overflow-x: initial !important;}:root { --bg-color:#ffffff; --text-color:#333333; --select-text-bg-color:#B5D6FC; --select-text-font-color:auto; --monospace:"Lucida Console",Consolas,"Courier",monospace; }
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kbd { margin: 0px 0.1em; padding: 0.1em 0.6em; font-size: 0.8em; color: rgb(36, 39, 41); background: rgb(255, 255, 255); border: 1px solid rgb(173, 179, 185); border-radius: 3px; box-shadow: rgba(12, 13, 14, 0.2) 0px 1px 0px, rgb(255, 255, 255) 0px 0px 0px 2px inset; white-space: nowrap; vertical-align: middle; }
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code { text-align: left; vertical-align: initial; }
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.MathJax_SVG * { transition: none 0s ease 0s; }
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.md-diagram-panel > svg { max-width: 100%; }
[lang="mermaid"] svg, [lang="flow"] svg { max-width: 100%; height: auto; }
[lang="mermaid"] .node text { font-size: 1rem; }
table tr th { border-bottom: 0px; }
video { max-width: 100%; display: block; margin: 0px auto; }
iframe { max-width: 100%; width: 100%; border: none; }
.highlight td, .highlight tr { border: 0px; }
svg[id^="mermaidChart"] { line-height: 1em; }
mark { background: rgb(255, 255, 0); color: rgb(0, 0, 0); }
.md-html-inline .md-plain, .md-html-inline strong, mark .md-inline-math, mark strong { color: inherit; }
mark .md-meta { color: rgb(0, 0, 0); opacity: 0.3 !important; }


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.CodeMirror-scroll { overflow: auto hidden; z-index: 3; }
.CodeMirror-gutter-filler, .CodeMirror-scrollbar-filler { background-color: rgb(255, 255, 255); }
.CodeMirror-gutters { border-right: 1px solid rgb(221, 221, 221); background: inherit; white-space: nowrap; }
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.cm-s-inner .cm-string { color: rgb(170, 17, 17); }
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.cm-s-inner .cm-string-2 { color: rgb(255, 85, 0); }
.cm-s-inner .cm-meta { color: rgb(85, 85, 85); }
.cm-s-inner .cm-qualifier { color: rgb(85, 85, 85); }
.cm-s-inner .cm-builtin { color: rgb(51, 0, 170); }
.cm-s-inner .cm-bracket { color: rgb(153, 153, 119); }
.cm-s-inner .cm-tag { color: rgb(17, 119, 0); }
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.cm-s-inner .cm-quote, .cm-s-inner.cm-quote { color: rgb(0, 153, 0); }
.cm-s-inner .cm-hr, .cm-s-inner.cm-hr { color: rgb(153, 153, 153); }
.cm-s-inner .cm-link, .cm-s-inner.cm-link { color: rgb(0, 0, 204); }
.cm-negative { color: rgb(221, 68, 68); }
.cm-positive { color: rgb(34, 153, 34); }
.cm-header, .cm-strong { font-weight: 700; }
.cm-del { text-decoration: line-through; }
.cm-em { font-style: italic; }
.cm-link { text-decoration: underline; }
.cm-error { color: red; }
.cm-invalidchar { color: red; }
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div.CodeMirror span.CodeMirror-nonmatchingbracket { color: rgb(255, 34, 34); }
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.CodeMirror-scrollbar-filler { right: 0px; bottom: 0px; }
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.CodeMirror-gutters { position: absolute; left: 0px; top: 0px; padding-bottom: 30px; z-index: 3; }
.CodeMirror-gutter { white-space: normal; height: 100%; box-sizing: content-box; padding-bottom: 30px; margin-bottom: -32px; display: inline-block; }
.CodeMirror-gutter-wrapper { position: absolute; z-index: 4; background: 0px 0px !important; border: none !important; }
.CodeMirror-gutter-background { position: absolute; top: 0px; bottom: 0px; z-index: 4; }
.CodeMirror-gutter-elt { position: absolute; cursor: default; z-index: 4; }
.CodeMirror-lines { cursor: text; }
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.CodeMirror-wrap pre { overflow-wrap: break-word; white-space: pre-wrap; word-break: normal; }
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/* Flowchart variables */
/* Sequence Diagram variables */
/* Gantt chart variables */
/* state colors */
.label {
  
  color: #333; }

.label text {
  fill: #333; }

.node rect,
.node circle,
.node ellipse,
.node polygon {
  fill: #BDD5EA;
  stroke: #9370DB;
  stroke-width: 1px; }

.node .label {
  text-align: center; }

.node.clickable {
  cursor: pointer; }

.arrowheadPath {
  fill: lightgrey; }

.edgePath .path {
  stroke: lightgrey;
  stroke-width: 1.5px; }

.edgeLabel {
  background-color: #e8e8e8;
  text-align: center; }

.cluster rect {
  fill: #6D6D65;
  stroke: rgba(255, 255, 255, 0.25);
  stroke-width: 1px; }

.cluster text {
  fill: #F9FFFE; }

div.mermaidTooltip {
  position: absolute;
  text-align: center;
  max-width: 200px;
  padding: 2px;
  
  font-size: 12px;
  background: #6D6D65;
  border: 1px solid rgba(255, 255, 255, 0.25);
  border-radius: 2px;
  pointer-events: none;
  z-index: 100; }

.actor {
  stroke: #81B1DB;
  fill: #BDD5EA; }

text.actor {
  fill: black;
  stroke: none; }

.actor-line {
  stroke: lightgrey; }

.messageLine0 {
  stroke-width: 1.5;
  stroke-dasharray: '2 2';
  stroke: lightgrey; }

.messageLine1 {
  stroke-width: 1.5;
  stroke-dasharray: '2 2';
  stroke: lightgrey; }

#arrowhead {
  fill: lightgrey; }

.sequenceNumber {
  fill: white; }

#sequencenumber {
  fill: lightgrey; }

#crosshead path {
  fill: lightgrey !important;
  stroke: lightgrey !important; }

.messageText {
  fill: lightgrey;
  stroke: none; }

.labelBox {
  stroke: #81B1DB;
  fill: #BDD5EA; }

.labelText {
  fill: #323D47;
  stroke: none; }

.loopText {
  fill: lightgrey;
  stroke: none; }

.loopLine {
  stroke-width: 2;
  stroke-dasharray: '2 2';
  stroke: #81B1DB; }

.note {
  stroke: rgba(255, 255, 255, 0.25);
  fill: #fff5ad; }

.noteText {
  fill: black;
  stroke: none;
  
  font-size: 14px; }

.activation0 {
  fill: #f4f4f4;
  stroke: #666; }

.activation1 {
  fill: #f4f4f4;
  stroke: #666; }

.activation2 {
  fill: #f4f4f4;
  stroke: #666; }

/** Section styling */
.section {
  stroke: none;
  opacity: 0.2; }

.section0 {
  fill: rgba(255, 255, 255, 0.3); }

.section2 {
  fill: #EAE8B9; }

.section1,
.section3 {
  fill: white;
  opacity: 0.2; }

.sectionTitle0 {
  fill: #F9FFFE; }

.sectionTitle1 {
  fill: #F9FFFE; }

.sectionTitle2 {
  fill: #F9FFFE; }

.sectionTitle3 {
  fill: #F9FFFE; }

.sectionTitle {
  text-anchor: start;
  font-size: 11px;
  text-height: 14px;
   }

/* Grid and axis */
.grid .tick {
  stroke: lightgrey;
  opacity: 0.3;
  shape-rendering: crispEdges; }

.grid path {
  stroke-width: 0; }

/* Today line */
.today {
  fill: none;
  stroke: #DB5757;
  stroke-width: 2px; }

/* Task styling */
/* Default task */
.task {
  stroke-width: 2; }

.taskText {
  text-anchor: middle;
   }

.taskText:not([font-size]) {
  font-size: 11px; }

.taskTextOutsideRight {
  fill: #323D47;
  text-anchor: start;
  font-size: 11px;
   }

.taskTextOutsideLeft {
  fill: #323D47;
  text-anchor: end;
  font-size: 11px; }

/* Special case clickable */
.task.clickable {
  cursor: pointer; }

.taskText.clickable {
  cursor: pointer;
  fill: #003163 !important;
  font-weight: bold; }

.taskTextOutsideLeft.clickable {
  cursor: pointer;
  fill: #003163 !important;
  font-weight: bold; }

.taskTextOutsideRight.clickable {
  cursor: pointer;
  fill: #003163 !important;
  font-weight: bold; }

/* Specific task settings for the sections*/
.taskText0,
.taskText1,
.taskText2,
.taskText3 {
  fill: #323D47; }

.task0,
.task1,
.task2,
.task3 {
  fill: #BDD5EA;
  stroke: rgba(255, 255, 255, 0.5); }

.taskTextOutside0,
.taskTextOutside2 {
  fill: lightgrey; }

.taskTextOutside1,
.taskTextOutside3 {
  fill: lightgrey; }

/* Active task */
.active0,
.active1,
.active2,
.active3 {
  fill: #81B1DB;
  stroke: rgba(255, 255, 255, 0.5); }

.activeText0,
.activeText1,
.activeText2,
.activeText3 {
  fill: #323D47 !important; }

/* Completed task */
.done0,
.done1,
.done2,
.done3 {
  stroke: grey;
  fill: lightgrey;
  stroke-width: 2; }

.doneText0,
.doneText1,
.doneText2,
.doneText3 {
  fill: #323D47 !important; }

/* Tasks on the critical line */
.crit0,
.crit1,
.crit2,
.crit3 {
  stroke: #E83737;
  fill: #E83737;
  stroke-width: 2; }

.activeCrit0,
.activeCrit1,
.activeCrit2,
.activeCrit3 {
  stroke: #E83737;
  fill: #81B1DB;
  stroke-width: 2; }

.doneCrit0,
.doneCrit1,
.doneCrit2,
.doneCrit3 {
  stroke: #E83737;
  fill: lightgrey;
  stroke-width: 2;
  cursor: pointer;
  shape-rendering: crispEdges; }

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.milestoneText {
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g.classGroup text {
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g.classGroup rect {
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g.classGroup line {
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.classLabel .label {
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.relation {
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  fill: none; }

#compositionStart {
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#compositionEnd {
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#aggregationStart {
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#aggregationEnd {
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#dependencyStart {
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#dependencyEnd {
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#extensionStart {
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#extensionEnd {
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.commit-id,
.commit-msg,
.branch-label {
  fill: lightgrey;
  color: lightgrey;
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.pieTitleText {
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  font-size: 25px;
  fill: #323D47;
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.slice {
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g.stateGroup text {
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g.stateGroup circle {
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}

g.stateGroup .state-title {
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g.stateGroup rect {
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g.stateGroup line {
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.transition {
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.stateGroup .composit {
  fill: #555;
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.state-note {
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.stateLabel .box {
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.stateLabel text {
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;
/* CSS Document */

/** code highlight */

.cm-s-inner .cm-variable,
.cm-s-inner .cm-operator,
.cm-s-inner .cm-property {
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}

.cm-s-inner .cm-keyword {
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}

.cm-s-inner .cm-tag {
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}

.cm-s-inner .cm-attribute {
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}

.CodeMirror div.CodeMirror-cursor {
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}

.cm-s-inner .cm-string {
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}

.cm-s-inner .cm-comment,
.cm-s-inner.cm-comment {
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}

.cm-s-inner .cm-header,
.cm-s-inner .cm-def,
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}

.cm-s-inner .cm-quote,
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.cm-s-inner .cm-hr {
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}

.cm-s-inner .cm-link {
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.cm-s-inner .cm-string-2 {
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.cm-s-inner .cm-meta,
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.cm-s-inner .cm-atom,
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.cm-s-inner .cm-number {
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.cm-s-inner .cm-variable {
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}

.cm-s-inner .cm-variable-2 {
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}

.cm-s-inner .cm-variable-3 {
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}

.CodeMirror-selectedtext,
.CodeMirror-selected {
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.CodeMirror-gutters {
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}
;
/* CSS Document */

/** markdown source **/
.cm-s-typora-default .cm-header, 
.cm-s-typora-default .cm-property
{
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}

.CodeMirror.cm-s-typora-default div.CodeMirror-cursor{
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}

.cm-s-typora-default .cm-comment {
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}

.cm-s-typora-default .cm-string {
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}

.cm-s-typora-default .cm-atom, .cm-s-typora-default .cm-number {
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}

.cm-s-typora-default .cm-link {
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.cm-s-typora-default .CodeMirror-activeline-background {
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}

.cm-s-typora-default .cm-comment, .cm-s-typora-default .cm-code {
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}@import "";
@import "";
@import "";

:root {
    --bg-color:  #363B40;
    --side-bar-bg-color: #2E3033;
    --text-color: #b8bfc6;

    --select-text-bg-color:#4a89dc;

    --item-hover-bg-color: #0a0d16;
    --control-text-color: #b7b7b7;
    --control-text-hover-color: #eee;
    --window-border: 1px solid #555;

    --active-file-bg-color: rgb(34, 34, 34);
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    --primary-color: #a3d5fe;

    --active-file-text-color: white;
    --item-hover-bg-color: #70717d;
    --item-hover-text-color: white;
    --primary-color: #6dc1e7;

    --rawblock-edit-panel-bd: #333;

    --search-select-bg-color: #428bca;
}

html {
    font-size: 16px;
}

html,
body {
    -webkit-text-size-adjust: 100%;
    -ms-text-size-adjust: 100%;
    background: #363B40;
    background: var(--bg-color);
    fill: currentColor;
    line-height: 1.625rem;
}

#write {
    max-width: 914px;
}

html,
body,
button,
input,
select,
textarea,
div.code-tooltip-content {
    color: #b8bfc6;
    border-color: transparent;
}

div.code-tooltip,
.md-hover-tip .md-arrow:after {
    background: #333;
}

.popover.bottom > .arrow:after {
    border-bottom-color: #333;
}

html,
body,
button,
input,
select,
textarea {
    font-family: "Helvetica Neue", Helvetica, Arial, sans-serif;
}

hr {
    height: 2px;
    border: 0;
    margin: 24px 0 !important;
}

h1,
h2,
h3,
h4,
h5,
h6 {
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    clear: both;
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    padding: 0;
    color: #DEDEDE
}

h1 {
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}

h2 {
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    /* 30px */
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}

h3 {
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}

h4 {
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}

h5 {
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}

h6 {
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}

@media (min-width: 980px) {
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    h4.md-focus:before,
    h5.md-focus:before,
    h6.md-focus:before {
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        border: 1px solid #ddd;
        border-radius: 3px;
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        left: -1.642857143rem;
        top: .357142857rem;
        float: left;
        font-size: 9px;
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        padding-right: 2px;
        vertical-align: bottom;
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    h3.md-focus:before {
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    h4.md-focus:before {
        content: 'h4';
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    h5.md-focus:before {
        content: 'h5';
        top: 0px;
    }

    h6.md-focus:before {
        content: 'h6';
        top: 0px;
    }
}

a {
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}

a:hover {
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}

a:focus {
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}

sup.md-footnote {
    background-color: #555;
    color: #ddd;
}

p {
    -ms-word-wrap: break-word;
    word-wrap: break-word;
}

p,
ul,
dd,
ol,
hr,
address,
pre,
table,
iframe,
.wp-caption,
.wp-audio-shortcode,
.wp-video-shortcode {
    margin-top: 0;
    margin-bottom: 1.5rem;
    /* 24px */
}

li > blockquote {
	margin-bottom: 0;
}

audio:not([controls]) {
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}

[hidden] {
    display: none;
}

::-moz-selection {
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}

*.in-text-selection,
::selection {
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    color: #fff;
    text-shadow: none;
}

ul,
ol {
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}

ul {
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}

ol {
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}

ul ul,
ol ol,
ul ol,
ol ul {
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}

b,
th,
dt,
strong {
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}

i,
em,
dfn,
cite {
    font-style: italic;
}

blockquote {
    padding-left: 1.875rem;
    margin: 0 0 1.875rem 1.875rem;
    border-left: solid 2px #474d54;
    padding-left: 30px;
    margin-top: 35px;
}

pre,
code,
kbd,
tt,
var {
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    font-family: Monaco, Consolas, "Andale Mono", "DejaVu Sans Mono", monospace;
}

kbd {
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    font-size: 90%;
    color: #fff;
    background-color: #333;
    border-radius: 3px;
    box-shadow: inset 0 -1px 0 rgba(0,0,0,.25);
}

pre.md-fences {
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    margin-bottom: 20px;
    background: #333;
}

.CodeMirror-gutters {
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}

.enable-diagrams pre.md-fences[lang="sequence"] .code-tooltip,
.enable-diagrams pre.md-fences[lang="flow"] .code-tooltip,
.enable-diagrams pre.md-fences[lang="mermaid"] .code-tooltip {
    bottom: -2.2em;
    right: 4px;
}

code,
kbd,
tt,
var {
    padding: 2px 5px;
}

table {
    max-width: 100%;
    width: 100%;
    border-collapse: collapse;
    border-spacing: 0;
}

th,
td {
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    vertical-align: top;
}

a {
    -webkit-transition: all .2s ease-in-out;
    transition: all .2s ease-in-out;
}

hr {
    background: #474d54;
    /* variable */
}

h1 {
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}

a {
    color: #e0e0e0;
    text-decoration: underline;
}

a:hover {
    color: #fff;
}

.md-inline-math script {
    color: #81b1db;
}

b,
th,
dt,
strong {
    color: #DEDEDE;
    /* variable */
}

mark {
    background: #D3D40E;
}

blockquote {
    color: #9DA2A6;
}

table a {
    color: #DEDEDE;
    /* variable */
}

th,
td {
    border: solid 1px #474d54;
    /* variable */
}

.task-list {
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}

.md-task-list-item {
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}

.md-task-list-item > input {
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}

.md-task-list-item > input:before {
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    width: 0.875rem;
    height: 0.875rem;
    vertical-align: middle;
    text-align: center;
    border: 1px solid #b8bfc6;
    background-color: #363B40;
    margin-top: -0.4rem;
}

.md-task-list-item > input:checked:before,
.md-task-list-item > input[checked]:before {
    content: '\221A';
    /*◘*/
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    line-height: 0.625rem;
    color: #DEDEDE;
}

/** quick open **/
.auto-suggest-container {
    border: 0px;
    background-color: #525C65;
}

#typora-quick-open {
    background-color: #525C65;
}

#typora-quick-open input{
    background-color: #525C65;
    border: 0;
    border-bottom: 1px solid grey;
}

.typora-quick-open-item {
    background-color: inherit;
    color: inherit;
}

.typora-quick-open-item.active,
.typora-quick-open-item:hover {
    background-color: #4D8BDB;
    color: white;
}

.typora-quick-open-item:hover {
    background-color: rgba(77, 139, 219, 0.8);
}

.typora-search-spinner > div {
  background-color: #fff;
}

#write pre.md-meta-block {
    border-bottom: 1px dashed #ccc;
    background: transparent;
    padding-bottom: 0.6em;
    line-height: 1.6em;
}

.btn,
.btn .btn-default {
    background: transparent;
    color: #b8bfc6;
}

.ty-table-edit {
    border-top: 1px solid gray;
    background-color: #363B40;
}

.popover-title {
    background: transparent;
}

.md-image>.md-meta {
    color: #BBBBBB;
    background: transparent;
}

.md-expand.md-image>.md-meta {
    color: #DDD;
}

#write>h3:before,
#write>h4:before,
#write>h5:before,
#write>h6:before {
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    border-radius: 0px;
    color: #888;
    text-decoration: underline;
    left: -1.4rem;
    top: 0.2rem;
}

#write>h3.md-focus:before {
    top: 2px;
}

#write>h4.md-focus:before {
    top: 2px;
}

.md-toc-item {
    color: #A8C2DC;
}

#write div.md-toc-tooltip {
    background-color: #363B40;
}

.dropdown-menu .btn:hover,
.dropdown-menu .btn:focus,
.md-toc .btn:hover,
.md-toc .btn:focus {
    color: white;
    background: black;
}

#toc-dropmenu {
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    border: 1px solid rgba(253, 253, 253, 0.15);
}

#toc-dropmenu .divider {
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}

.outline-expander:before {
    top: 2px;
}

#typora-sidebar {
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    border-right: none;
}

.sidebar-tabs {
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}

#typora-sidebar:hover .outline-title-wrapper {
    border-left: 1px dashed;
}

.outline-title-wrapper .btn {
    color: inherit;
}

.outline-item:hover {
    border-color: #363B40;
    background-color: #363B40;
    color: white;
}

h1.md-focus .md-attr,
h2.md-focus .md-attr,
h3.md-focus .md-attr,
h4.md-focus .md-attr,
h5.md-focus .md-attr,
h6.md-focus .md-attr,
.md-header-span .md-attr {
    color: #8C8E92;
    display: inline;
}

.md-comment {
    color: #5a95e3;
    opacity: 1;
}

.md-inline-math svg {
    color: #b8bfc6;
}

#math-inline-preview .md-arrow:after {
    background: black;
}

.modal-content {
    background: var(--bg-color);
    border: 0;
}

.modal-title {
    font-size: 1.5em;
}

.modal-content input {
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    color: white;
}

.modal-content .input-group-addon {
    color: white;
}

.modal-backdrop {
    background-color: rgba(174, 174, 174, 0.7);
}

.modal-content .btn-primary {
    border-color: var(--primary-color);
}

.md-table-resize-popover {
    background-color: #333;
}

.form-inline .input-group .input-group-addon {
    color: white;
}

#md-searchpanel {
    border-bottom: 1px dashed grey;
}

/** UI for electron */

.context-menu,
#spell-check-panel,
#footer-word-count-info {
    background-color: #42464A;
}

.context-menu.dropdown-menu .divider,
.dropdown-menu .divider {
    background-color: #777777;
}

footer {
    color: inherit;
}

@media (max-width: 1000px) {
    footer {
        border-top: none;
    }
    footer:hover {
        color: inherit;
    }
}

#file-info-file-path .file-info-field-value:hover {
    background-color: #555;
    color: #dedede;
}

.megamenu-content,
.megamenu-opened header {
    background: var(--bg-color);
}

.megamenu-menu-panel h2,
.megamenu-menu-panel h1,
.long-btn {
    color: inherit;
}

.megamenu-menu-panel input[type='text'] {
    background: inherit;
    border: 0;
    border-bottom: 1px solid;
}

#recent-file-panel-action-btn {
    background: inherit;
    border: 1px grey solid;
}

.megamenu-menu-panel .dropdown-menu > li > a {
    color: inherit;
    background-color: #2F353A;
    text-decoration: none;
}

.megamenu-menu-panel table td:nth-child(1) {
    color: inherit;
    font-weight: bold;
}

.megamenu-menu-panel tbody tr:hover td:nth-child(1) {
    color: white;
}

.modal-footer .btn-default, 
.modal-footer .btn-primary,
.modal-footer .btn-default:not(:hover) {
    border: 1px solid;
    border-color: transparent;
}

.btn-default:hover, .btn-default:focus, .btn-default.focus, .btn-default:active, .btn-default.active, .open > .dropdown-toggle.btn-default {
    color: white;
    border: 1px solid #ddd;
    background-color: inherit;
}

.modal-header {
    border-bottom: 0;
}

.modal-footer {
    border-top: 0;
}

#recent-file-panel tbody tr:nth-child(2n-1) {
    background-color: transparent !important;
}

.megamenu-menu-panel tbody tr:hover td:nth-child(2) {
    color: inherit;
}

.megamenu-menu-panel .btn {
    border: 1px solid #eee;
    background: transparent;
}

.mouse-hover .toolbar-icon.btn:hover,
#w-full.mouse-hover,
#w-pin.mouse-hover {
    background-color: inherit;
}

.typora-node::-webkit-scrollbar {
    width: 5px;
}

.typora-node::-webkit-scrollbar-thumb:vertical {
    background: rgba(250, 250, 250, 0.3);
}

.typora-node::-webkit-scrollbar-thumb:vertical:active {
    background: rgba(250, 250, 250, 0.5);
}

#w-unpin {
    background-color: #4182c4;
}

#top-titlebar, #top-titlebar * {
    color: var(--item-hover-text-color);
}

.typora-sourceview-on #toggle-sourceview-btn,
#footer-word-count:hover,
.ty-show-word-count #footer-word-count {
    background: #333333;
}

#toggle-sourceview-btn:hover {
    color: #eee;
    background: #333333;
}

/** focus mode */
.on-focus-mode .md-end-block:not(.md-focus):not(.md-focus-container) * {
    color: #686868 !important;
}

.on-focus-mode .md-end-block:not(.md-focus) img,
.on-focus-mode .md-task-list-item:not(.md-focus-container)>input {
    opacity: #686868 !important;
}

.on-focus-mode li[cid]:not(.md-focus-container){
    color: #686868;
}

.on-focus-mode .md-fences.md-focus .CodeMirror-code>*:not(.CodeMirror-activeline) *,
.on-focus-mode .CodeMirror.cm-s-inner:not(.CodeMirror-focused) * {
    color: #686868 !important;
}

.on-focus-mode .md-focus,
.on-focus-mode .md-focus-container {
    color: #fff;
}

.on-focus-mode #typora-source .CodeMirror-code>*:not(.CodeMirror-activeline) * {
    color: #686868 !important;
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<div  id='write'  class = 'is-node'><h3><a name="1约束是什么" class="md-header-anchor"></a><span>1、约束是什么</span></h3><pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang="sql"><div class="CodeMirror cm-s-inner CodeMirror-wrap" lang="sql"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 0px; left: 4px;"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" tabindex="0" style="position: absolute; bottom: -1em; padding: 0px; width: 1000px; height: 1em; outline: none;"></textarea></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 0px; margin-bottom: 0px; border-right-width: 0px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><pre><span>xxxxxxxxxx</span></pre></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation" style=""><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: 0px; width: 0px;"></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">创建表的时候可以给表的字段添加相应的约束，目的是为了保证表中数据的合法性、有效性以及完整性。</span></pre></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">--常见约束有：</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-tab" role="presentation" cm-text="	">    </span>非空约束（not <span class="cm-atom">null</span>）：约束的字段中数据不能为null</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-tab" role="presentation" cm-text="	">    </span>唯一约束（unique）：约束的字段中数据不能重复</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-tab" role="presentation" cm-text="	">    </span>主键约束（primary key）：约束的字段中的数据既不能为null也不能重复，简称PK</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-tab" role="presentation" cm-text="	">    </span>外键约束（foreign key）：...简称FK</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-tab" role="presentation" cm-text="	">    </span>检查约束（check）：Oracle数据库中有该约束，MySQL中目前还没有</span></pre></div></div></div></div></div><div style="position: absolute; height: 0px; width: 1px; border-bottom: 0px solid transparent; top: 183px;"></div><div class="CodeMirror-gutters" style="display: none; height: 183px;"></div></div></div></pre><h3><a name="2非空约束not-null）" class="md-header-anchor"></a><span>2、非空约束（not null）</span></h3><pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang="sql" style="break-inside: unset;"><div class="CodeMirror cm-s-inner CodeMirror-wrap" lang="sql"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 0px; left: 4px;"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" tabindex="0" style="position: absolute; bottom: -1em; padding: 0px; width: 1000px; height: 1em; outline: none;"></textarea></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 0px; margin-bottom: 0px; border-right-width: 0px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><pre><span>xxxxxxxxxx</span></pre></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation" style=""><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: 0px; width: 0px;"></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-keyword">drop</span> <span class="cm-keyword">table</span> if exists t_user;</span></pre></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-keyword">create</span> <span class="cm-keyword">table</span> t_user(</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-tab" role="presentation" cm-text="	">    </span>id <span class="cm-builtin">int</span>,</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp;  username <span class="cm-builtin">varchar</span>(<span class="cm-number">255</span>) <span class="cm-keyword">not</span> <span class="cm-atom">null</span>,</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp;  password <span class="cm-builtin">varchar</span>(<span class="cm-number">255</span>)</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">);</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">mysql&gt; <span class="cm-keyword">drop</span> <span class="cm-keyword">table</span> if exists t_user;</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">Query OK, <span class="cm-number">0</span> rows affected, <span class="cm-number">1</span> warning (<span class="cm-number">0.01</span> sec)</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">mysql&gt; <span class="cm-keyword">create</span> <span class="cm-keyword">table</span> t_user(</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp;  -&gt; id <span class="cm-builtin">int</span>,</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp;  -&gt; &nbsp; &nbsp; username <span class="cm-builtin">varchar</span>(<span class="cm-number">255</span>) <span class="cm-keyword">not</span> <span class="cm-atom">null</span>,</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp;  -&gt; &nbsp; &nbsp; password <span class="cm-builtin">varchar</span>(<span class="cm-number">255</span>)</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp;  -&gt; );</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">Query OK, <span class="cm-number">0</span> rows affected (<span class="cm-number">0.40</span> sec)</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">mysql&gt; <span class="cm-keyword">desc</span> t_user;</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">+----------+--------------+------+-----+---------+-------+</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">| Field &nbsp;  | Type &nbsp; &nbsp; &nbsp; &nbsp; | <span class="cm-atom">Null</span> | Key | Default | Extra |</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">+----------+--------------+------+-----+---------+-------+</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">| id &nbsp; &nbsp; &nbsp; | <span class="cm-builtin">int</span>(<span class="cm-number">11</span>) &nbsp; &nbsp;  | YES  | &nbsp; &nbsp; | <span class="cm-atom">NULL</span> &nbsp;  | &nbsp; &nbsp; &nbsp; |</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">| username | <span class="cm-builtin">varchar</span>(<span class="cm-number">255</span>) | NO &nbsp; | &nbsp; &nbsp; | <span class="cm-atom">NULL</span> &nbsp;  | &nbsp; &nbsp; &nbsp; |</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">| password | <span class="cm-builtin">varchar</span>(<span class="cm-number">255</span>) | YES  | &nbsp; &nbsp; | <span class="cm-atom">NULL</span> &nbsp;  | &nbsp; &nbsp; &nbsp; |</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">+----------+--------------+------+-----+---------+-------+</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-number">3</span> rows <span class="cm-keyword">in</span> <span class="cm-keyword">set</span> (<span class="cm-number">0.00</span> sec)<span class="cm-comment">--可以看到表中的username中的null是NO</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-keyword">insert</span> <span class="cm-keyword">into</span> t_user (id,password) <span class="cm-keyword">values</span> (<span class="cm-number">1</span>,<span class="cm-string">'123'</span>);<span class="cm-comment">--报错，因为username不能为null，该字段不添加数据底层又无法给其一个null，但是又需要赋值，报没有设定默认值的错误</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">mysql&gt; <span class="cm-keyword">insert</span> <span class="cm-keyword">into</span> t_user (id,password) <span class="cm-keyword">values</span> (<span class="cm-number">1</span>,<span class="cm-string">'123'</span>);</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">ERROR <span class="cm-number">1364</span> (HY000): Field <span class="cm-string">'username'</span> doesn<span class="cm-string">'t have a default value</span></span></pre></div></div></div></div></div><div style="position: absolute; height: 0px; width: 1px; border-bottom: 0px solid transparent; top: 769px;"></div><div class="CodeMirror-gutters" style="display: none; height: 769px;"></div></div></div></pre><h3><a name="2唯一性约束unique）" class="md-header-anchor"></a><span>2、唯一性约束（unique）</span></h3><p><span>unique修饰的字段不可以重复，但可以为null且多个null</span></p><pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang="sql" style="break-inside: unset;"><div class="CodeMirror cm-s-inner CodeMirror-wrap" lang="sql"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 0px; left: 4px;"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" tabindex="0" style="position: absolute; bottom: -1em; padding: 0px; width: 1000px; height: 1em; outline: none;"></textarea></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 0px; margin-bottom: 0px; border-right-width: 0px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><pre><span>xxxxxxxxxx</span></pre></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation" style=""><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: 0px; width: 0px;"></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-keyword">drop</span> <span class="cm-keyword">table</span> if exists t_user;</span></pre></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-keyword">create</span> <span class="cm-keyword">table</span> t_user(</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-tab" role="presentation" cm-text="	">    </span>id <span class="cm-builtin">int</span>,</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp;  username <span class="cm-builtin">varchar</span>(<span class="cm-number">255</span>) unique,</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp;  password <span class="cm-builtin">varchar</span>(<span class="cm-number">255</span>)</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">);</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-keyword">insert</span> <span class="cm-keyword">into</span> t_user <span class="cm-keyword">values</span>(<span class="cm-number">1</span>,<span class="cm-string">'zz'</span>,<span class="cm-string">'123'</span>);</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-keyword">insert</span> <span class="cm-keyword">into</span> t_user <span class="cm-keyword">values</span>(<span class="cm-number">2</span>,<span class="cm-string">'zz'</span>,<span class="cm-string">'123'</span>);<span class="cm-comment">--报错</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">mysql&gt; <span class="cm-keyword">create</span> <span class="cm-keyword">table</span> t_user(</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp;  -&gt; id <span class="cm-builtin">int</span>,</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp;  -&gt; &nbsp; &nbsp; username <span class="cm-builtin">varchar</span>(<span class="cm-number">255</span>) unique,</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp;  -&gt; &nbsp; &nbsp; password <span class="cm-builtin">varchar</span>(<span class="cm-number">255</span>)</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp;  -&gt; );</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">Query OK, <span class="cm-number">0</span> rows affected (<span class="cm-number">0.34</span> sec)</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">mysql&gt; <span class="cm-keyword">insert</span> <span class="cm-keyword">into</span> t_user <span class="cm-keyword">values</span>(<span class="cm-number">1</span>,<span class="cm-string">'zz'</span>,<span class="cm-string">'123'</span>);</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">Query OK, <span class="cm-number">1</span> row affected (<span class="cm-number">0.06</span> sec)</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">mysql&gt; <span class="cm-keyword">insert</span> <span class="cm-keyword">into</span> t_user <span class="cm-keyword">values</span>(<span class="cm-number">2</span>,<span class="cm-string">'zz'</span>,<span class="cm-string">'123'</span>);</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">ERROR <span class="cm-number">1062</span> (<span class="cm-number">23000</span>): Duplicate entry <span class="cm-string">'zz'</span> for key <span class="cm-string">'username'</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">--但可以这样</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-keyword">insert</span> <span class="cm-keyword">into</span> t_user(id) <span class="cm-keyword">values</span>(<span class="cm-number">3</span>);</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-keyword">insert</span> <span class="cm-keyword">into</span> t_user(id) <span class="cm-keyword">values</span>(<span class="cm-number">4</span>);</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">mysql&gt; <span class="cm-keyword">select</span> * <span class="cm-keyword">from</span> t_user;</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">+------+----------+----------+</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">| id &nbsp; | username | password |</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">+------+----------+----------+</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">| &nbsp; &nbsp;<span class="cm-number">1</span> | zz &nbsp; &nbsp; &nbsp; | <span class="cm-number">123</span> &nbsp; &nbsp;  |</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">| &nbsp; &nbsp;<span class="cm-number">3</span> | <span class="cm-atom">NULL</span> &nbsp; &nbsp; | <span class="cm-atom">NULL</span> &nbsp; &nbsp; |</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">| &nbsp; &nbsp;<span class="cm-number">4</span> | <span class="cm-atom">NULL</span> &nbsp; &nbsp; | <span class="cm-atom">NULL</span> &nbsp; &nbsp; |</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">+------+----------+----------+</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-number">3</span> rows <span class="cm-keyword">in</span> <span class="cm-keyword">set</span> (<span class="cm-number">0.00</span> sec)</span></pre></div></div></div></div></div><div style="position: absolute; height: 0px; width: 1px; border-bottom: 0px solid transparent; top: 871px;"></div><div class="CodeMirror-gutters" style="display: none; height: 871px;"></div></div></div></pre><h3><a name="3给两个列或多个列字段添加unique" class="md-header-anchor"></a><span>3、给两个列或多个列字段添加unique</span></h3><pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang="sql" style="break-inside: unset;"><div class="CodeMirror cm-s-inner CodeMirror-wrap" lang="sql"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 0px; left: 4px;"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" tabindex="0" style="position: absolute; bottom: -1em; padding: 0px; width: 1000px; height: 1em; outline: none;"></textarea></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 0px; margin-bottom: 0px; border-right-width: 0px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><pre><span>xxxxxxxxxx</span></pre></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation" style=""><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: 0px; width: 0px;"></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-keyword">drop</span> <span class="cm-keyword">table</span> if exists t_user;</span></pre></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-keyword">create</span> <span class="cm-keyword">table</span> t_user(</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-tab" role="presentation" cm-text="	">    </span>id <span class="cm-builtin">int</span>,</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp;  username <span class="cm-builtin">varchar</span>(<span class="cm-number">255</span>),</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp;  password <span class="cm-builtin">varchar</span>(<span class="cm-number">255</span>),</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp;  unique(username,password)<span class="cm-comment">--表级约束</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">);</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">--上面这种方式表示username和password这两个字段的数据联合拼接出来的字符不能出现重复，联合添加约束</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">--可以写成这样</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-keyword">insert</span> <span class="cm-keyword">into</span> t_user <span class="cm-keyword">values</span>(<span class="cm-number">1</span>,<span class="cm-string">'zz'</span>,<span class="cm-string">'111'</span>);</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-keyword">insert</span> <span class="cm-keyword">into</span> t_user <span class="cm-keyword">values</span>(<span class="cm-number">2</span>,<span class="cm-string">'zz'</span>,<span class="cm-string">'222'</span>);</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">--下面这种方式表示username和password单独插入数据不能重复，是分别添加约束</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-keyword">drop</span> <span class="cm-keyword">table</span> if exists t_user;</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-keyword">create</span> <span class="cm-keyword">table</span> t_user(</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-tab" role="presentation" cm-text="	">    </span>id <span class="cm-builtin">int</span>,</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp;  username <span class="cm-builtin">varchar</span>(<span class="cm-number">255</span>) unique,</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp;  password <span class="cm-builtin">varchar</span>(<span class="cm-number">255</span>) unique<span class="cm-comment">--列级约束</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">);</span></pre></div></div></div></div></div><div style="position: absolute; height: 0px; width: 1px; border-bottom: 0px solid transparent; top: 488px;"></div><div class="CodeMirror-gutters" style="display: none; height: 488px;"></div></div></div></pre><h3><a name="4主键约束" class="md-header-anchor"></a><span>4、主键约束</span></h3><h5><a name="给一张表添加主键" class="md-header-anchor"></a><span>给一张表添加主键</span></h5><pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang="sql" style="break-inside: unset;"><div class="CodeMirror cm-s-inner CodeMirror-wrap" lang="sql"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 0px; left: 4px;"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" tabindex="0" style="position: absolute; bottom: -1em; padding: 0px; width: 1000px; height: 1em; outline: none;"></textarea></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 0px; margin-bottom: 0px; border-right-width: 0px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><pre><span>xxxxxxxxxx</span></pre></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation" style=""><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: 0px; width: 0px;"></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-keyword">drop</span> <span class="cm-keyword">table</span> if exists t_user;</span></pre></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-keyword">create</span> <span class="cm-keyword">table</span> t_user(</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-tab" role="presentation" cm-text="	">    </span>id <span class="cm-builtin">int</span> primary key,<span class="cm-comment">--列级约束</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp;  username <span class="cm-builtin">varchar</span>(<span class="cm-number">255</span>),</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp;  password <span class="cm-builtin">varchar</span>(<span class="cm-number">255</span>)</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">);</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-keyword">insert</span> <span class="cm-keyword">into</span> t_user(id,username,password) <span class="cm-keyword">values</span>(<span class="cm-number">1</span>,<span class="cm-string">'zs'</span>,<span class="cm-string">'@zs.com'</span>);</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-keyword">insert</span> <span class="cm-keyword">into</span> t_user(id,username,password) <span class="cm-keyword">values</span>(<span class="cm-number">2</span>,<span class="cm-string">'ls'</span>,<span class="cm-string">'@ls.com'</span>);</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">mysql&gt; <span class="cm-keyword">select</span> * <span class="cm-keyword">from</span> t_user;</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">+----+----------+----------+</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">| id | username | password |</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">+----+----------+----------+</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">| &nbsp;<span class="cm-number">1</span> | zs &nbsp; &nbsp; &nbsp; | @zs<span class="cm-variable-2">.com</span>  |</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">| &nbsp;<span class="cm-number">2</span> | ls &nbsp; &nbsp; &nbsp; | @ls<span class="cm-variable-2">.com</span>  |</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">+----+----------+----------+</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-number">2</span> rows <span class="cm-keyword">in</span> <span class="cm-keyword">set</span> (<span class="cm-number">0.00</span> sec)</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">--下面插入数据出现错误</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-keyword">insert</span> <span class="cm-keyword">into</span> t_user(id,username,password) <span class="cm-keyword">values</span>(<span class="cm-number">1</span>,<span class="cm-string">'ww'</span>,<span class="cm-string">'@ww.com'</span>);</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">mysql&gt; <span class="cm-keyword">insert</span> <span class="cm-keyword">into</span> t_user(id,username,password) <span class="cm-keyword">values</span>(<span class="cm-number">1</span>,<span class="cm-string">'ww'</span>,<span class="cm-string">'@ww.com'</span>);</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">ERROR <span class="cm-number">1062</span> (<span class="cm-number">23000</span>): Duplicate entry <span class="cm-string">'1'</span> for key <span class="cm-string">'PRIMARY'</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">由上面可知：id字段是主键字段，因为添加了主键约束primary key，所以该字段中的数据既不能为null，也不能重复。这是主键字段的特点。</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">--主键相关术语</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">主键字段：被主键约束primary key修饰的字段</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">主键值：主键字段中的值</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">主键约束：primary key</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">--主键的作用</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-tab" role="presentation" cm-text="	">    </span>表的设计三范式中有要求，第一范式就要求每张表中必须有主键。</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-tab" role="presentation" cm-text="	">    </span>作用：主键值是这张表中的某行记录的唯一标识，就是说无论哪两行记录的除了主键字段外的所有字段的值都相同，只要主键值不同，这就是两行不同的记录（就像人的身份证号一样）</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" class="cm-tab-wrap-hack" style="padding-right: 0.1px;"><span class="cm-tab" role="presentation" cm-text="	">    </span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">--主键的分类</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-tab" role="presentation" cm-text="	">    </span>根据主键字段的数量来划分：</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-tab" role="presentation" cm-text="	">    </span><span class="cm-tab" role="presentation" cm-text="	">    </span>单一主键（推荐且是常用）</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-tab" role="presentation" cm-text="	">    </span><span class="cm-tab" role="presentation" cm-text="	">    </span>复合主键（多个字段联合起来使用一个主键）-不建议使用，违反三范式原则</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-tab" role="presentation" cm-text="	">    </span>根据主键性质划分：</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-tab" role="presentation" cm-text="	">    </span><span class="cm-tab" role="presentation" cm-text="	">    </span>自然主键：主键值最好是一个于业务没有任何关系的自然数</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-tab" role="presentation" cm-text="	">    </span><span class="cm-tab" role="presentation" cm-text="	">    </span>业务主键：主键值和系统的业务挂钩，例如用银行卡号或者身份证号做主键值，这是不推荐使用的。</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-tab" role="presentation" cm-text="	">    </span><span class="cm-tab" role="presentation" cm-text="	">    </span>原因是往后业务一旦改变，这些主键也需要随之变动，但是主键变动可能会导致主键重复，但主键是不能重复的，这<span class="cm-tab" role="presentation" cm-text="	"> </span><span class="cm-tab" role="presentation" cm-text="	">    </span>对于业务影响较大。</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" class="cm-tab-wrap-hack" style="padding-right: 0.1px;"><span class="cm-tab" role="presentation" cm-text="	">    </span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">--一张表中只能有一个主键约束</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">--使用表级约束定义主键</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-keyword">drop</span> <span class="cm-keyword">table</span> if exists t_user;</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-keyword">create</span> <span class="cm-keyword">table</span> t_user(</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-tab" role="presentation" cm-text="	">    </span>id <span class="cm-builtin">int</span> ,</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp;  username <span class="cm-builtin">varchar</span>(<span class="cm-number">255</span>),</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp;  password <span class="cm-builtin">varchar</span>(<span class="cm-number">255</span>),</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp;  primary key(id)</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">);</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">--mysql提供主键值自增，插入数据时没有给主键字段数据主键会自动按照主键值递增的方式赋值</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">--id字段自动维护一个自增的数字，从1开始，以1递增</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-keyword">drop</span> <span class="cm-keyword">table</span> if exists t_user;</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-keyword">create</span> <span class="cm-keyword">table</span> t_user(</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-tab" role="presentation" cm-text="	">    </span>id <span class="cm-builtin">int</span> primary key auto_increment,</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp;  username <span class="cm-builtin">varchar</span>(<span class="cm-number">255</span>),</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp;  password <span class="cm-builtin">varchar</span>(<span class="cm-number">255</span>)</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">);</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre></div></div></div></div></div><div style="position: absolute; height: 0px; width: 1px; border-bottom: 0px solid transparent; top: 1650px;"></div><div class="CodeMirror-gutters" style="display: none; height: 1650px;"></div></div></div></pre><h3><a name="5外键约束-foreign--key" class="md-header-anchor"></a><span>5、外键约束-foreign  key</span></h3><pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang="sql" style="break-inside: unset;"><div class="CodeMirror cm-s-inner CodeMirror-wrap" lang="sql"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 0px; left: 4px;"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" tabindex="0" style="position: absolute; bottom: -1em; padding: 0px; width: 1000px; height: 1em; outline: none;"></textarea></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 0px; margin-bottom: 0px; border-right-width: 0px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><pre><span>xxxxxxxxxx</span></pre></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation" style=""><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: 0px; width: 0px;"></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">--外键相关术语</span></span></pre></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-tab" role="presentation" cm-text="	">    </span>外键约束：foreign key</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-tab" role="presentation" cm-text="	">    </span>外键字段：添加有外键约束的字段</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-tab" role="presentation" cm-text="	">    </span>外键值：外键字段的每一个值</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">--业务背景</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-tab" role="presentation" cm-text="	">    </span>设计数据库表，用来维护学生和班级的信息</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">第一种方案：设计一张表存储所有数据，缺点是太冗余了</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">第二种方案：设计两张表，学生表和班级表</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">t_class 班级表</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">cno(PK)<span class="cm-tab" role="presentation" cm-text="	"> </span><span class="cm-tab" role="presentation" cm-text="	">    </span><span class="cm-tab" role="presentation" cm-text="	">    </span>cname</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">-----------------------------------</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-number">101</span><span class="cm-tab" role="presentation" cm-text="	"> </span><span class="cm-tab" role="presentation" cm-text="	">    </span><span class="cm-tab" role="presentation" cm-text="	">    </span>西电1602052</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-number">102</span><span class="cm-tab" role="presentation" cm-text="	"> </span><span class="cm-tab" role="presentation" cm-text="	">    </span><span class="cm-tab" role="presentation" cm-text="	">    </span>西电1602051</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">-----------------------------------</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">t_student 学生表</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">sno(PK)<span class="cm-tab" role="presentation" cm-text="	"> </span><span class="cm-tab" role="presentation" cm-text="	">    </span><span class="cm-tab" role="presentation" cm-text="	">    </span>sname<span class="cm-tab" role="presentation" cm-text="	">   </span><span class="cm-tab" role="presentation" cm-text="	">    </span>classno(FK)</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">-----------------------------------------</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-number">1</span><span class="cm-tab" role="presentation" cm-text="	">   </span><span class="cm-tab" role="presentation" cm-text="	">    </span><span class="cm-tab" role="presentation" cm-text="	">    </span><span class="cm-tab" role="presentation" cm-text="	">    </span>zs<span class="cm-tab" role="presentation" cm-text="	">  </span><span class="cm-tab" role="presentation" cm-text="	">    </span><span class="cm-tab" role="presentation" cm-text="	">    </span><span class="cm-number">101</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-number">2</span><span class="cm-tab" role="presentation" cm-text="	">   </span><span class="cm-tab" role="presentation" cm-text="	">    </span><span class="cm-tab" role="presentation" cm-text="	">    </span><span class="cm-tab" role="presentation" cm-text="	">    </span>ls<span class="cm-tab" role="presentation" cm-text="	">  </span><span class="cm-tab" role="presentation" cm-text="	">    </span><span class="cm-tab" role="presentation" cm-text="	">    </span><span class="cm-number">102</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-number">3</span><span class="cm-tab" role="presentation" cm-text="	">   </span><span class="cm-tab" role="presentation" cm-text="	">    </span><span class="cm-tab" role="presentation" cm-text="	">    </span><span class="cm-tab" role="presentation" cm-text="	">    </span>ww<span class="cm-tab" role="presentation" cm-text="	">  </span><span class="cm-tab" role="presentation" cm-text="	">    </span><span class="cm-tab" role="presentation" cm-text="	">    </span><span class="cm-number">102</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-number">4</span><span class="cm-tab" role="presentation" cm-text="	">   </span><span class="cm-tab" role="presentation" cm-text="	">    </span><span class="cm-tab" role="presentation" cm-text="	">    </span><span class="cm-tab" role="presentation" cm-text="	">    </span>cl<span class="cm-tab" role="presentation" cm-text="	">  </span><span class="cm-tab" role="presentation" cm-text="	">    </span><span class="cm-tab" role="presentation" cm-text="	">    </span><span class="cm-number">101</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">-----------------------------------------</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">--由于设计两张表，我们可能不知道学生是哪个班级的，所以将学生表中的classno字段添加一个外键约束，用班级表中的cno字段(要求该字段是主键字段)里的数据来约束classno字段要填的值，即classno字段值只能填班级表cno字段中的值，填其他值会报错。这样就可以知道学生是哪个班级了。</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">--实现上面的建表语句</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">由于学生表中的某个字段只能填另一个表中的字段的值，填其他会报错，所以就有父表和子表之分，且建表、插入数据以及删表的顺序有先后</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">t_student中的classno字段引用t_class表中的cno字段，此时t_student叫做子表，t_class表叫做父表</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">--顺序如下</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">删除数据，先删子表，再删父表</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">添加数据，先添加父表，后添加子表</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">删除表，先删子表，再删父表</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">创建表，先创父表，后创子表</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">--references 表示引用的意思</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-keyword">drop</span> <span class="cm-keyword">table</span> if exists t_student;</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-keyword">drop</span> <span class="cm-keyword">table</span> if exists t_class;</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-keyword">create</span> <span class="cm-keyword">table</span> t_class(</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-tab" role="presentation" cm-text="	">    </span>cno <span class="cm-builtin">int</span>,</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp;  cname <span class="cm-builtin">varchar</span>(<span class="cm-number">255</span>),</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp;  primary key(cno)</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">);</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-keyword">create</span> <span class="cm-keyword">table</span> t_student(</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-tab" role="presentation" cm-text="	">    </span>sno <span class="cm-builtin">int</span>,</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp;  sname <span class="cm-builtin">varchar</span>(<span class="cm-number">255</span>),</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp;  classno <span class="cm-builtin">int</span>,</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp;  foreign key (classno) references t_class(cno)</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">);</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-keyword">insert</span> <span class="cm-keyword">into</span> t_class <span class="cm-keyword">values</span>(<span class="cm-number">101</span>,<span class="cm-string">'xxxxxxxxx'</span>);</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-keyword">insert</span> <span class="cm-keyword">into</span> t_class <span class="cm-keyword">values</span>(<span class="cm-number">102</span>,<span class="cm-string">'yyyyyyyyy'</span>);</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-keyword">insert</span> <span class="cm-keyword">into</span> t_student <span class="cm-keyword">values</span>(<span class="cm-number">1</span>,<span class="cm-string">'zs'</span>,<span class="cm-number">101</span>);</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-keyword">insert</span> <span class="cm-keyword">into</span> t_student <span class="cm-keyword">values</span>(<span class="cm-number">2</span>,<span class="cm-string">'ls'</span>,<span class="cm-number">102</span>);</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-keyword">insert</span> <span class="cm-keyword">into</span> t_student <span class="cm-keyword">values</span>(<span class="cm-number">3</span>,<span class="cm-string">'ww'</span>,<span class="cm-number">102</span>);</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-keyword">insert</span> <span class="cm-keyword">into</span> t_student <span class="cm-keyword">values</span>(<span class="cm-number">4</span>,<span class="cm-string">'cl'</span>,<span class="cm-number">101</span>);</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-keyword">select</span> * <span class="cm-keyword">from</span> t_class;</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-keyword">select</span> * <span class="cm-keyword">from</span> t_student;</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">--将上面的sql语句放进记事本改为sql脚本就可以了</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">--由于开始没有将班级表中的cno设为主键，在创建学生表时无法添加外键约束导致了建学生表失败</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">mysql&gt; <span class="cm-keyword">drop</span> <span class="cm-keyword">table</span> if exists t_student;</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">Query OK, <span class="cm-number">0</span> rows affected (<span class="cm-number">0.24</span> sec)</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">mysql&gt; <span class="cm-keyword">drop</span> <span class="cm-keyword">table</span> if exists t_class;</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">Query OK, <span class="cm-number">0</span> rows affected (<span class="cm-number">0.13</span> sec)</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">mysql&gt; <span class="cm-keyword">create</span> <span class="cm-keyword">table</span> t_class(</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp;  -&gt; cno <span class="cm-builtin">int</span>,</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp;  -&gt; &nbsp; &nbsp; cname <span class="cm-builtin">varchar</span>(<span class="cm-number">255</span>),</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp;  -&gt; &nbsp; &nbsp; primary key(cno)</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp;  -&gt; );</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">Query OK, <span class="cm-number">0</span> rows affected (<span class="cm-number">0.26</span> sec)</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">mysql&gt; <span class="cm-keyword">create</span> <span class="cm-keyword">table</span> t_student(</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp;  -&gt; sno <span class="cm-builtin">int</span>,</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp;  -&gt; &nbsp; &nbsp; sname <span class="cm-builtin">varchar</span>(<span class="cm-number">255</span>),</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp;  -&gt; &nbsp; &nbsp; classno <span class="cm-builtin">int</span>,</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp;  -&gt; &nbsp; &nbsp; foreign key (classno) references t_class(cno)</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp;  -&gt; );</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">Query OK, <span class="cm-number">0</span> rows affected (<span class="cm-number">0.28</span> sec)</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">mysql&gt; <span class="cm-keyword">insert</span> <span class="cm-keyword">into</span> t_class <span class="cm-keyword">values</span>(<span class="cm-number">101</span>,<span class="cm-string">'xxxxxxxxx'</span>);</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">Query OK, <span class="cm-number">1</span> row affected (<span class="cm-number">0.12</span> sec)</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">mysql&gt; <span class="cm-keyword">insert</span> <span class="cm-keyword">into</span> t_class <span class="cm-keyword">values</span>(<span class="cm-number">102</span>,<span class="cm-string">'yyyyyyyyy'</span>);</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">Query OK, <span class="cm-number">1</span> row affected (<span class="cm-number">0.03</span> sec)</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">mysql&gt; <span class="cm-keyword">insert</span> <span class="cm-keyword">into</span> t_student <span class="cm-keyword">values</span>(<span class="cm-number">1</span>,<span class="cm-string">'zs'</span>,<span class="cm-number">101</span>);</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">Query OK, <span class="cm-number">1</span> row affected (<span class="cm-number">0.08</span> sec)</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">mysql&gt; <span class="cm-keyword">insert</span> <span class="cm-keyword">into</span> t_student <span class="cm-keyword">values</span>(<span class="cm-number">2</span>,<span class="cm-string">'ls'</span>,<span class="cm-number">102</span>);</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">Query OK, <span class="cm-number">1</span> row affected (<span class="cm-number">0.01</span> sec)</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">mysql&gt; <span class="cm-keyword">insert</span> <span class="cm-keyword">into</span> t_student <span class="cm-keyword">values</span>(<span class="cm-number">3</span>,<span class="cm-string">'ww'</span>,<span class="cm-number">102</span>);</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">Query OK, <span class="cm-number">1</span> row affected (<span class="cm-number">0.09</span> sec)</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">mysql&gt; <span class="cm-keyword">insert</span> <span class="cm-keyword">into</span> t_student <span class="cm-keyword">values</span>(<span class="cm-number">4</span>,<span class="cm-string">'cl'</span>,<span class="cm-number">101</span>);</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">Query OK, <span class="cm-number">1</span> row affected (<span class="cm-number">0.03</span> sec)</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">mysql&gt; <span class="cm-keyword">select</span> * <span class="cm-keyword">from</span> t_class;</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">+-----+-----------+</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">| cno | cname &nbsp; &nbsp; |</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">+-----+-----------+</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">| <span class="cm-number">101</span> | xxxxxxxxx |</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">| <span class="cm-number">102</span> | yyyyyyyyy |</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">+-----+-----------+</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-number">2</span> rows <span class="cm-keyword">in</span> <span class="cm-keyword">set</span> (<span class="cm-number">0.00</span> sec)</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">mysql&gt; <span class="cm-keyword">select</span> * <span class="cm-keyword">from</span> t_student;</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">+------+-------+---------+</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">| sno  | sname | classno |</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">+------+-------+---------+</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">| &nbsp; &nbsp;<span class="cm-number">1</span> | zs &nbsp;  | &nbsp; &nbsp; <span class="cm-number">101</span> |</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">| &nbsp; &nbsp;<span class="cm-number">2</span> | ls &nbsp;  | &nbsp; &nbsp; <span class="cm-number">102</span> |</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">| &nbsp; &nbsp;<span class="cm-number">3</span> | ww &nbsp;  | &nbsp; &nbsp; <span class="cm-number">102</span> |</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">| &nbsp; &nbsp;<span class="cm-number">4</span> | cl &nbsp;  | &nbsp; &nbsp; <span class="cm-number">101</span> |</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">+------+-------+---------+</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-number">4</span> rows <span class="cm-keyword">in</span> <span class="cm-keyword">set</span> (<span class="cm-number">0.00</span> sec)</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">--这时我们往t_student表中插入数据，当classno字段不是班级表中主键字段cno中的值时报错</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-keyword">insert</span> <span class="cm-keyword">into</span> t_student <span class="cm-keyword">values</span>(<span class="cm-number">5</span>,<span class="cm-string">'dw'</span>,<span class="cm-number">103</span>);</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">mysql&gt; <span class="cm-keyword">insert</span> <span class="cm-keyword">into</span> t_student <span class="cm-keyword">values</span>(<span class="cm-number">5</span>,<span class="cm-string">'dw'</span>,<span class="cm-number">103</span>);<span class="cm-comment">--子记录外键约束添加失败</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">ERROR <span class="cm-number">1452</span> (<span class="cm-number">23000</span>): Cannot add <span class="cm-keyword">or</span> <span class="cm-keyword">update</span> a child row: a foreign key constraint fails (`mysqlstudy`.`t_student`, CONSTRAINT `t_student_ibfk_1` FOREIGN KEY (`classno`) REFERENCES `t_class` (`cno`))</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">--外键值是可以为null的</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">--外键字段引用其他表中的某个字段时，被引用的字段不一定是主键字段，但是至少具有unique约束，就是说不能重复</span></span></pre></div></div></div></div></div><div style="position: absolute; height: 0px; width: 1px; border-bottom: 0px solid transparent; top: 3314px;"></div><div class="CodeMirror-gutters" style="display: none; height: 3314px;"></div></div></div></pre><p>&nbsp;</p></div>
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